Porous media have been the backbone of almost all dissipative noise abatement techniques. This may be a mature and reliable technique which works for a very broad frequency band. But there may be two drawbacks.
Firstly, existing sound absorption techniques may be ineffective in the low frequency range, such as that below 200 Hz. This range is also often over-looked due to its low A-weighting. Nevertheless, the actual power of noise radiation from, say, a fan, is often highest in this frequency range.
Secondly, there has been increasing concern about the deposition and accumulation of dusts in the pores of the porous material. A periodical cleaning of the lining would be rather costly and indeed tedious. In fact, there are already public concerns of bacteria breading in the centralized ventilation systems of ordinary commercial buildings. The use of porous material for noise or heat insulation purposes might have contributed to an indoor air quality which is often worse than outdoors. There are also places such as operation theatres, where high hygienic requirement forbids the use of such materials.
The need of controlling low frequency noise in an environment friendly manner calls for a fiberless approach. A team of acousticians led by Fuchs (2001a) utilized microperforated sheets for sound absorption, and impervious thin membranes for separating the harsh environment from acoustic elements like resonators (Fuchs 2001b ). Active control techniques are also implemented, albeit in the simplest and most practical fashion. The construction of such microperforated sheets may be relatively complicated leading to high fabrication and maintenance costs.